Driving method for an electrophoretic display with high frame rate and low peak power consumption

ABSTRACT

An image is updated on a bi-stable display (310) such as an electrophoretic display by using voltage waveforms ( 600, 620, 640, 660; 700, 720, 740, 760; 800, 820, 840, 860 ) that are configured such that voltage changes are constrained to a subset of possible voltage levels during specific frame times. The specific frame times may occur during datadependent portions of the waveforms, such as a reset portion (R) and/or a drive portion (D, D 1 , D 2 ). Due to the reduced voltage swing, the supply voltage can be reduced, resulting in reduced power consumption. Moreover, the frame time (FT′) can be shortened during the data-dependent portions of the waveforms to increase the greyscale accuracy and number of grey levels. At other frames times, the voltage levels can vary throughout the full range of possible voltage levels, while a standard frame time (FT) is used.

The invention relates generally to electronic reading devices such aselectronic books and electronic newspapers and, more particularly, to amethod and apparatus for providing set of driving waveforms for drivinga bi-stable display such as an electrophoretic display while reducingpower consumption.

Recent technological advances have provided “user friendly” electronicreading devices such as e-books that open up many opportunities. Forexample, electrophoretic displays hold much promise. Such displays havean intrinsic memory behavior and are able to hold an image for arelatively long time without power consumption. Power is consumed onlywhen the display needs to be refreshed or updated with new information.So, the power consumption in such displays is very low, suitable forapplications for portable e-reading devices like e-books ande-newspaper. Electrophoresis refers to movement of charged particles inan applied electric field. When electrophoresis occurs in a liquid, theparticles move with a velocity determined primarily by the viscous dragexperienced by the particles, their charge (either permanent orinduced), the dielectric properties of the liquid, and the magnitude ofthe applied field. An electrophoretic display is a type of bi-stabledisplay, which is a display that substantially holds an image withoutconsuming power after an image update.

For example, international patent application WO 99/53373, publishedApr. 9, 1999, by E Ink Corporation, Cambridge, Mass., US, and entitledFull Color Reflective Display With Multichromatic Sub-Pixels, describessuch a display device. WO 99/53373 discusses an electronic ink displayhaving two substrates. One is transparent, and the other is providedwith electrodes arranged in rows and columns. A display element or pixelis associated with an intersection of a row electrode and columnelectrode. The display element is coupled to the column electrode usinga thin film transistor (TFT), the gate of which is coupled to the rowelectrode. This arrangement of display elements, TFT transistors, androw and column electrodes together forms an active matrix. Furthermore,the display element comprises a pixel electrode. A row driver selects arow of display elements, and a column or source driver supplies a datasignal to the selected row of display elements via the column electrodesand the TFT transistors. The data signals correspond to graphic data tobe displayed, such as text or figures.

The electronic ink is provided between the pixel electrode and a commonelectrode on the transparent substrate. The electronic ink comprisesmultiple microcapsules of about 10 to 50 microns in diameter. In oneapproach, each microcapsule has positively charged white particles andnegatively charged black particles suspended in a liquid carrier mediumor fluid. When a positive voltage is applied to the pixel electrode, thewhite particles move to a side of the microcapsule directed to thetransparent substrate and a viewer will see a white display element. Atthe same time, the black particles move to the pixel electrode at theopposite side of the microcapsule where they are hidden from the viewer.By applying a negative voltage to the pixel electrode, the blackparticles move to the common electrode at the side of the microcapsuledirected to the transparent substrate and the display element appearsdark to the viewer. At the same time, the white particles move to thepixel electrode at the opposite side of the microcapsule where they arehidden from the viewer. When the voltage is removed, the display deviceremains in the acquired state and thus exhibits a bi-stable character.In another approach, particles are provided in a dyed liquid.

For example, black particles may be provided in a white liquid, or whiteparticles may be provided in a black liquid. Or, other colored particlesmay be provided in different colored liquids, e.g., white particles inblue liquid.

Other fluids such as air may also be used in the medium in which thecharged black and white particles move around in an electric field(e.g., Bridgestone SID2003—Symposium on Information Displays. May 18-23,2003, —digest 20.3). Colored particles may also be used.

To form an electronic display, the electronic ink may be printed onto asheet of plastic film that is laminated to a layer of circuitry. Thecircuitry forms a pattern of pixels that can then be controlled by adisplay driver. Since the microcapsules are suspended in a liquidcarrier medium, they can be printed using existing screen-printingprocesses onto virtually any surface, including glass, plastic, fabricand even paper. Moreover, the use of flexible sheets allows the designof electronic reading devices that approximate the appearance of aconventional book.

However, the power consumed by the electronic display can becomeunacceptably high due to large, rapid changes in the voltage applied tothe pixels as a frame is updated, especially with higher frame rates.Higher frame rates may be used at higher temperatures, or to increasethe number of grey levels or the greyscale accuracy, for instance.

The invention addresses the above and other issues by providing a methodand apparatus for providing set of driving waveforms for driving abi-stable display such as an electrophoretic display while reducingpower consumption.

In a particular aspect of the invention, a method provides a set ofvoltage waveforms for updating at least a portion of a bi-stable displayin successive frame periods. The method includes accessing data definingthe set of voltage waveforms for the successive frame periods, andgenerating the set of voltage waveforms for driving the at least aportion of the bi-stable display during the successive frame periodsaccording to the accessed data Over a duration of the successive frameperiods, each of the voltage waveforms spans a first range of values.Moreover, at least one of the successive frame periods is time-alignedwith a data-dependent portion of each of the voltage waveforms thatspans a second range of values that is a subset of the first range ofvalues.

A related electronic reading device and program storage device are alsoprovided.

In the drawings:

FIG. 1 shows diagramatically a front view of an embodiment of a portionof a display screen of an electronic reading device;

FIG. 2 shows diagramatically a cross-sectional view along 2-2 in FIG. 1;

FIG. 3 shows diagramatically an overview of an electronic readingdevice;

FIG. 4 shows diagramatically two display screens with respective displayregions;

FIG. 5 illustrates example waveforms for image transitions where a highpeak power is expected between t0 and t1, and between t1 and t2;

FIG. 6 illustrates example waveforms for image transitions where a partof the drive pulse in the transition from B to G2 is delayed by threeframe periods, in accordance with a first embodiment of the invention;

FIG. 7 illustrates example waveforms for image transitions where a partof the drive pulse in the transitions from W to G1, and from G2 to G1,is delayed by two frame periods, in accordance with a second embodimentof the invention; and

FIG. 8 illustrates example waveforms for image transitions where a partof the drive pulse in the transitions from W to G1, and from G2 to G1,is delayed by three frame periods, in accordance with a third embodimentof the invention.

In all the Figures, corresponding parts are referenced by the samereference numerals.

Each of the following is incorporated herein by reference:

European patent application EP 02078823.8, entitled “ElectrophoreticDisplay Panel”, filed Sep. 16, 2002 (docket no. PHNL 020844);

European patent application EP 03100133.2, entitled “Electrophoreticdisplay panel”, filed Jan. 23, 2003 (docket no. PHNL 030091);

European patent application EP 02077017.8, entitled “Display Device”,filed May 24, 2002, or WO 03/079323, Electrophoretic Active MatrixDisplay Device”, published Feb. 6, 2003 (docket no. PHNL 020441); and

European patent application EP 03101705.6, entitled “ElectrophoreticDisplay Unit”, filed Jun. 11, 2003 (docket no. PHNL 030661).

FIGS. 1 and 2 show the embodiment of a portion of a display panel 1 ofan electronic reading device having a first substrate 8, a secondopposed substrate 9 and a plurality of picture elements 2. The pictureelements 2 may be arranged along substantially straight lines in atwo-dimensional structure. The picture elements 2 are shown spaced apartfrom one another for clarity, but in practice, the picture elements 2are very close to one another so as to form a continuous image.Moreover, only a portion of a full display screen is shown. Otherarrangements of the picture elements are possible, such as a honeycombarrangement. An electrophoretic medium 5 having charged particles 6 ispresent between the substrates 8 and 9. A first electrode 3 and secondelectrode 4 are associated with each picture element 2. The electrodes 3and 4 are able to receive a potential difference. In FIG. 2, for eachpicture element 2, the first substrate has a first electrode 3 and thesecond substrate 9 has a second electrode 4. The charged particles 6 areable to occupy positions near either of the electrodes 3 and 4 orintermediate to them. Each picture element 2 has an appearancedetermined by the position of the charged particles 6 between theelectrodes 3 and 4. Electrophoretic media 5 are known per se, e.g., fromU.S. Pat. Nos. 5,961,804, 6,120,839, and 6,130,774 and can be obtained,for instance, from E Ink Corporation.

As an example, the electrophoretic medium 5 may contain negativelycharged black particles 6 in a white fluid. When the charged particles 6are near the first electrode 3 due to a potential difference of, e.g.,+15 Volts, the appearance of the picture elements 2 is white. When thecharged particles 6 are near the second electrode 4 due to a potentialdifference of opposite polarity, e.g., −15 Volts, the appearance of thepicture elements 2 is black. When the charged particles 6 are betweenthe electrodes 3 and 4, the picture element has an intermediateappearance such as a grey level between black and white. Anapplication-specific integrated circuit (ASIC) 100 controls thepotential difference of each picture element 2 to create a desiredpicture, e.g. images and/or text, in a full display screen.

The full display screen is made up of numerous picture elements thatcorrespond to pixels in a display.

FIG. 3 shows diagramatically an overview of an electronic readingdevice. The electronic reading device 300 includes the display ASIC 100.For example, the ASIC 100 may be the Philips Corp. “Apollo” ASIC E-inkdisplay controller. The display ASIC 100 controls the one or moredisplay screens 310, such as electrophoretic screens, via an addressingcircuit 305, to cause desired text or images to be displayed. Theaddressing circuit 305 includes driving integrated circuits (ICs). Forexample, the display ASIC 100 may act as a voltage source that providesvoltage waveforms, via an addressing circuit 305, to the differentpixels in the display screen 310. The addressing circuit 305 providesinformation for addressing specific pixels, such as row and column, tocause the desired image or text to be displayed. The display ASIC 100causes successive pages to be displayed starting on different rowsand/or columns. The image or text data may be stored in a memory 320,which represents one or more storage devices, and accessed by the ASIC100 as needed. One example is the Philips Electronics small form factoroptical (SFFO) disk system, in other systems a non-volatile flash memorycould be utilized. The electronic reading device 300 further includes areading device controller 330 or host controller, which may beresponsive to a user-activated software or hardware button 322 thatinitiates a user command such as a next page command or previous pagecommand.

The reading device controller 330 may be part of a computer thatexecutes any type of computer code devices, such as software, firmware,micro code or the like, to achieve the functionality described herein.Accordingly, a computer program product comprising such computer codedevices may be provided in a manner apparent to those skilled in the artThe reading device controller 330 may further comprise a memory (notshown) that is a program storage device that tangibly embodies a programof instructions executable by a machine such as the reading devicecontroller 330 or a computer to perform a method that achieves thefunctionality described herein. Such a program storage device may beprovided in a manner apparent to those skilled in the art.

The display ASIC 100 may have logic for periodically providing a forcedreset of a display region of an electronic book, e.g., after every xpages are displayed, after every y minutes, e.g., ten minutes, when theelectronic reading device 300 is first turned on, and/or when thebrightness deviation is larger than a value such as 3% reflection. Forautomatic resets, an acceptable frequency can be determined empiricallybased on the lowest frequency that results in acceptable image quality.Also, the reset can be initiated manually by the user via a functionbutton or other interface device, e.g., when the user starts to read theelectronic reading device, or when the image quality drops to anunacceptable level.

The ASIC 100 provides instructions to the display addressing circuit 305for driving the display 310 by accessing information stored in thememory 320.

The invention may be used with any type of electronic reading device.FIG. 4 illustrates one possible example of an electronic reading device400 having two separate display screens. Specifically, a first displayregion 442 is provided on a first screen 440, and a second displayregion 452 is provided on a second screen 450. The screens 440 and 450may be connected by a binding 445 that allows the screens to be foldedflat against each other, or opened up and laid flat on a surface. Thisarrangement is desirable since it closely replicates the experience ofreading a conventional book.

Various user interface devices may be provided to allow the user toinitiate page forward, page backward commands and the like. For example,the first region 442 may include on-screen buttons 424 that can beactivated using a mouse or other pointing device, a touch activation,PDA pen, or other known technique, to navigate among the pages of theelectronic reading device. In addition to page forward and page backwardcommands, a capability may be provided to scroll up or down in the samepage. Hardware buttons 422 may be provided alternatively, oradditionally, to allow the user to provide page forward and pagebackward commands. The second region 452 may also include on-screenbuttons 414 and/or hardware buttons 412. Note that the frame around thefirst and second display regions 442, 452 is not required as the displayregions may be frameless. Other interfaces, such as a voice commandinterface, may be used as well. Note that the buttons 412, 414; 422, 424are not required for both display regions. That is, a single set of pageforward and page backward buttons may be provided. Or, a single buttonor other device, such as a rocker switch, may be actuated to provideboth page forward and page backward commands. A function button or otherinterface device can also be provided to allow the user to manuallyinitiate a reset.

In other possible designs, an electronic book has a single displayscreen with a single display region that displays one page at a time.Or, a single display screen may be partitioned into or two or moredisplay regions arranged, e.g., horizontally or vertically. Furthermore,when multiple display regions are used, successive pages can bedisplayed in any desired order. For example, in FIG. 4, a first page canbe displayed on the display region 442, while a second page is displayedon the display region 452. When the user requests to view the next page,a third page may be displayed in the first display region 442 in placeof the first page while the second page remains displayed in the seconddisplay region 452. Similarly, a fourth page may be displayed in thesecond display region 452, and so forth. In another approach, when theuser requests to view the next page, both display regions are updated sothat the third page is displayed in the first display region 442 inplace of the first page, and the fourth page is displayed in the seconddisplay region 452 in place of the second page. When a single displayregion is used, a first page may be displayed, then a second pageoverwrites the first page, and so forth, when the user enters a nextpage command. The process can work in reverse for page back commands.Moreover, the process is equally applicable to languages in which textis read from right to left, such as Hebrew, as well as to languages suchas Chinese in which text is read column-wise rather than row-wise.

Additionally, note that the entire page need not be displayed on thedisplay region. A portion of the page may be displayed and a scrollingcapability provided to allow the user to scroll up, down, left or rightto read other portions of the page. A magnification and reductioncapability may be provided to allow the user to change the size of thetext or images. This may be desirable for users with reduced vision, forexample.

Problem Addressed Pulse width-modulation (PWM) has been found to be alow cost technique for driving a bi-stable display such as anelectrophoretic display, because of the low price of the drivers. Usinga driving waveform, the greyscale accuracy is limited by the minimumframe time, which is usually a standard time of 20 ms. However, ashorter frame time of about 8 ms has recently been achieved.

A bi-stable display such as an electrophoretic display is based on themotion of charged particles under an external electric field. Theswitching time is temperature dependent because of the change of theparticle mobility and/or the viscosity of the fluid with temperature.With present E-ink materials, the switching time decreases withincreasing temperature, and the driving voltage waveforms developed forroom temperature must be extended to the higher temperatures. A possibleapproach is to reduce the frame time (as discussed in European patentapplication EP 02078823.8, docket no. PHNL020844) by, for example,scaling, where a very short frame time is requested. In addition, astill shorter frame time is needed to achieve an increased number ofgrey levels, and to further improve the greyscale accuracy. However, theuse of a relatively short frame time results in higher powerconsumption. In particular, when the source driver integrated circuit(IC) has to operate in a full range of voltage values in the same shortframe scanning, an unacceptably high peak power will be requested. Theinvention addresses this problem.

Proposed Solution A technique is discussed for reducing powerconsumption in a bi-stable device while achieving an accurate greyscale,increasing the number of greyscale levels, while using a high framerate.

In one possible approach, driving waveforms for various greyscale imagetransitions are intentionally aligned in time such that voltage changesare constrained to a subset range of possible voltage values during oneor more frames. In other words, a full range voltage swings betweenmaximum and minimum values are avoided. For example, when the range ofpossible voltages is between −15 V and +15 V in the waveforms,variations from −15 V to +15 V, or from +15 V to −15 V, are avoided forspecific portions of the waveforms. Instead, variations between −15 Vand 0 V, or between 0 V and +15 V, are allowed for the specific portionsof the voltage waveforms. These waveform portions may includedata-dependent portions of the waveform in which a relatively shorterframe period is used. By reducing the voltage swing or span within oneor more frames, power consumption is significantly reduced. Inparticular, the peak power consumed by a bi-stable device isproportional to the square voltage-change, i.e., P∝C×(ΔV)², where Cdenotes capacitance. More specifically, the peak power consumed is theproduct of the capacitance×frequency×voltage swing×supply voltage. Thesupply voltage for the IC or chip that supplies voltage to pixels in thebi-stable device, such as in the addressing circuit 305, must be atleast equal to the voltage swing and may be 30 V, for example. Thevoltage swing or span is the range of possible voltages used, e.g., 30 V(+15 V-(−15V)). Thus, reducing the voltage swing by half, to 15V,reduces power consumption by half during specific frames.

However, in accordance with one aspect of the invention, the supplyvoltage can be reduced according to the reduced voltage swing, to e.g.,15 V. This reduces power consumption to one-fourth its original amount.As a result of the reduced supply voltage and voltage swing, a frametime of as short as one-fourth of the standard frame time may be usedwhile maintaining the same low power consumption. This is importantsince the availability of the short frame time is particularly useful inimproving the greyscale accuracy at higher temperatures and forincreasing the number of grey levels.

The invention is applicable to any driving scheme, includingrail-stabilized driving schemes, in which the driving pulses includereset pulses and greyscale driving pulses. A reset pulse is a voltagepulse that moves particles in the bi-stable display to one of the twoextreme optical states, and the greyscale driving pulse is a voltagepulse that sends the display/pixel to the desired final optical state.In the following embodiments, the rail-stabilized driving as disclosedin the above-referenced European patent application EP 03100133.2(docket no. PHNL030091) is used for explaining possible implementationsof the invention, although other driving schemes may be used.

FIG. 5 illustrates example waveforms for image transitions where a highpeak power is expected between t0 and t1, and between t1 and t2. Examplewaveforms are illustrated for image transitions from White (W) to Darkgrey (G1) (waveform 500), Black (B) to Light grey (G2) (waveform 520),G2 to G1 (waveform 540) and G2 to G2 (waveform 560) usingrail-stabilized driving. These examples represent a subset of thesixteen waveforms that are required to update an image for anelectrophoretic display with the four indicated brightness levels. Withrail-stabilized driving, a reset pulse (R) is used that has a durationthat is proportional to the distance that the particles need to movebetween two electrodes, e.g., from the white state (W) to the blackstate (SB), in waveform 500. The reset pulse (R) may have an over-resetduration that is used for improving the image quality. Over-reset pulsesare discussed in the above-referenced co-pending European patentapplication 03100133.2 (docket no. PHNL030091). In waveform 500, thesubsequent drive pulse (D) has an energy that is sufficient to drive thedisplay from the black state (SB) to the final state, which is the darkgrey (G1) state. The energy of a pulse is the product of voltageamplitude and duration.

Generally, a number of such waveforms are stored in memory in theelectronic device and used for driving the pixels in the display. Thewaveforms can be used for updating a portion of the display such as oneor more pixels, or the entire display. The vertical lines indicate frameboundaries. The frame time or period is the time between frameboundaries, or the inverse of the frame rate, which can vary in thewaveforms. The waveforms generally start and terminate at the same time.As discussed, a shorter frame time of, e.g., 8-10 ms, can be used forselected portions of the waveform, e.g., to increase accuracy andprovide more greyscale levels, while a longer, standard frame time of,e.g., 20 ms, may be used for other portions of the waveforms.

Each waveform 500, 520, 540 and 560 includes four portions: firstshaking pulses (S1), a reset portion (R), second shaking pulses (S2) anda drive portion (D). SB and SW denote a black or white state,respectively, reached via a reset pulse. Both the first and secondshaking pulses can be implemented by data-independent “hardware”shaking, where all pixels on the display receive the shaking signalsimultaneously, independent of data on individual pixels. This can beseen in that the shaking pulses S1 and S2 are time aligned among thedifferent waveforms. With hardware shaking, the power consumption can beminimized. Shaking pulses are discussed in the above-referencedco-pending European patent application 02077017.8 or WO 03/079324(docket no. PHNL020441). Integration of shaking pulses and over-resetpulses in the drive waveforms significantly improves the greyscaleaccuracy.

However, the reset (R) and drive pulses (D) are examples ofdata-dependent portions of the waveforms 500, 520, 540 and 560 sincethey are supplied to individual pixels, frame by frame, and thereforedepend on the data that defines the image of a frame. The reset pulse(R) is less sensitive to the choice of the frame time than the greyscaledrive pulse (D).

In fact, the greyscale drive pulse (D) is extremely sensitive to thechoice of the frame time because the greyscale accuracy in each imagetransition (e.g., W to G1, B to G2, etc.) is mainly determined by thedrive pulse frame time. We therefore focus on the drive portion in thefollowing discussion. However, the choice of frame time is important toany data dependent portion of a voltage waveform that is applied to abi-stable display.

In the image transitions of FIG. 5, the greyscale drive pulse (D) timeperiod (to) varies from two to five frame times or periods. Inparticular, for waveform 500, t_(D5)=five FT, including two standardframe times (FT) and three short frame times (FT′). For waveform 520,t_(D4)=four FT, including two standard frame times (FT) and two shortframe times (FT′). For waveform 540, t_(D3)=three FT, including twostandard frame times (FT) and one short frame time (FT′). For waveform560, t_(D2)=two FT, including two standard frame times (FT).

However, for some time-aligned frames, some of the greyscale drivepulses have positive voltages and others have negative voltages. Eachdrive portion or pulse (D) includes two standard frame times (FT), whichalready have a relatively low power consumption (although the sourcedriver operates at negative and positive voltages). One can evenconsider these frames as a single long “standard” frame using a singlescan, keeping the power consumption even lower. In the frame periodsbetween t₀ and t₁, and between t₁ and t₂, a single scan with the minimum(short) frame time FT′ is required during which both negative andpositive voltages have to be supplied by the source driver, resulting inan unacceptably high peak power. For example, between t₀ and t₁, thewaveforms 500, 520, 540 and 560 request −15 V, +15 V, −15 V, and 0 V,respectively.

Since the minimum and maximum voltages, −15 V and +15 V, respectively,are applied in the same frame, when updating different pixels, thevoltage source must switch between its minimum and maximum outputs whenaddressing the different pixels in the same frame, resulting in highpower consumption. The waveforms discussed below address this problem byavoiding a full range voltage swing in one or more specific frames forspecific data-dependent portions of the voltage waveforms that arealigned in time, e.g., in the same one or more frames.

FIG. 6 illustrates example waveforms for image transitions where a partof the drive pulse in the transition from B to G2 is delayed by threeframe periods, in accordance with a first embodiment of the invention.Waveform 600 provides an image transition from White (W) to Dark grey(G1), waveform 620 provides an image transition from Black (B) to Lightgrey (G2), waveform 640 provides an image transition from G2 to G1, andwaveform 660 provides an image transition from G2 to G2. The samewaveforms as used in FIG. 5 are presented, but now part of the greyscaledrive pulse in the B to G2 transition (waveform 620) is delayed by threeshort frames (FT′). In particular, the drive portion of waveform 620includes first and second drive portions D1 and D2, respectively, whereD2 follows D1 after the delay.

Between t₀ and t₁, a single scan with a frame time FT′ is used. However,now the voltage levels applied to different pixels in a common frame donot vary throughout the full (first) range of −15 V to +15 V. Instead,the voltage levels only vary in the subset (second) range of −15 V to 0V. Specifically, between t₀ and t₁, the waveforms 600, 620, 640 and 660request −15 V, 0 V, −15 V and 0 V. Similarly, in the frame time betweent1 and t₂, and between t₂ and t₃, the waveforms 600, 620, 640 and 660request −15 V, 0 V, 0 V, and 0 V.

Again, the voltage levels only vary in the subset range of −15 V to 0 V.In the frame time between t₃ and t₄, the waveforms 600, 620, 640 and 660request 0 V, +15 V, 0 V, and 0 V.

Here, the voltage levels only vary in the subset (third) range of 0 Vand +15 V. In fact, for each of the short frame times (FT′), the voltageswing is constrained to one of the subset voltage ranges.

In the examples discussed, the possible voltage values varied between aminimum of −15 V and a maximum of +15 V, where an intermediate value of0 V is also used.

However, the invention can be used with any range of voltages, and thevoltage range need not be centered about zero. For example, the minimumand maximum voltages might both have positive values, e.g., from +10 Vto +40 V. Moreover, it is possible to constrain the voltage levels totwo or more subset ranges within the range of possible values. Thesubset ranges may be contiguous, noncontiguous, and/or overlapping. Forexample, two subset ranges may be used, e.g., −15 V to 0 V, and 0 V to+15 V, that are contiguous and that span a first range of values of−15 Vto +15 V. Since the range of voltage values that is applied to thepixels is reduced, the supply voltage to the voltage source can also bereduced, as discussed, resulting in reduced power consumption for thespecific frame times. FIG. 7 illustrates example waveforms for imagetransitions where a part of the drive pulse in the transitions from W toG1, and from G2 to G1, is delayed by two frame periods, in accordancewith a second embodiment of the invention. Waveform 700 provides animage transition from White (W) to Dark grey (G1), waveform 720 providesan image transition from Black (B) to Light grey (G2), waveform 740provides an image transition from G2 to G1, and waveform 760 provides animage transition from G2 to G2. The same waveforms as used in FIG. 5 arepresented, but now part of the greyscale drive pulses (D2) in both the Wto G1 transition (waveform 700) and the G2 to G1 transition (waveform740) is delayed by two frames. In particular, for the waveform 700, thedrive portion includes a first drive portion (D1), followed by a delayof one or more frame times, followed by a second drive portion (D2). Thewaveform 740 similarly includes first and second drive portions D1 andD2, respectively. Again, for the short frame time (FT′) frames, thewaveforms are configured so that the voltage levels vary only within asubset range of possible values. For example, between t₀ and t₁, andbetween t₁ and t₂, the voltage levels only vary between 0 V and +15 Vsince the waveforms 700, 720, 740 and 760 request 0 V, +15 V, 0 V and 0V, respectively.

Between t₂ and t₃, the voltage levels only vary between −15 V and 0 Vsince the waveforms 700, 720, 740 and 760 request −15 V, 0 V, −15 V and0 V, respectively. Between t₃ and 4, and between t₄ and t₅, the voltagelevels only vary between −15 V and 0 V since the waveforms 700, 720, 740and 760 request −15 V, 0 V, 0 V and 0 V, respectively.

In the third and fourth frame (between to and t₂), two scans, each witha frame time of FT′, or one scan with a frame time of 2 FT′ may be used.In the fifth frame, between t₂ and t₃, a single scan with the minimum FTis used. In the sixth and seventh frames, between t3 and t5, two scans,each with a frame time of FT′, or one scan with a frame time of 2 FT′may be used.

FIG. 8 illustrates example waveforms for image transitions where a partof the drive pulse in the transitions from W to G1, and from G2 to G1,is delayed by three frame periods, in accordance with a third embodimentof the invention. Waveform 800 provides an image transition from White(W) to Dark grey (G1), waveform 820 provides an image transition fromBlack (B) to Light grey (G2), waveform 840 provides an image transitionfrom G2 to G1, and waveform 860 provides an image transition from G2 toG2. The third embodiment is derived from the second embodiment, but anadditional frame is added. In particular, relative to the correspondingwaveforms of FIG. 7, an additional frame with V=0 is used after thecompletion of the positive drive pulse in the B to G2 transition(waveform 800) and the G2 to G2 transition (waveform 860), and prior tothe start of the negative drive pulses (D2) in the W to G1 and G2 to G1transitions (waveforms 800 and 840, respectively). In particular, inwaveform 800, the second portion of the drive pulse (D2) is delayed bythree frames instead of two frames from the first portion of the drivepulse (D1).

In waveform 820, an additional frame with V=0 is provided following thedrive portion (D).

In waveform 840, the second portion of the drive pulse (D2) is delayedby three frames instead of two frames from the first portion of thedrive pulse (D1). In waveform 860, an additional frame with V=0 isprovided following the drive portion (D). This approach may furtherreduce the time-averaged load of the source driver, thereby furtherreducing time-averaged power consumption.

Note that, in the above examples, pulse-width modulated (PWM) driving isused for illustrating the invention, where the pulse time is varied ineach waveform while the voltage amplitude is kept constant. However, theinvention is also applicable to other driving schemes, e.g., based onvoltage modulated driving (VM), where the pulse voltage amplitude isvaried in each waveform, or combined PWM and VM driving. The inventionis applicable to color as well as greyscale bi-stable displays. Also,the electrode structure is not limited. For example, a top/bottomelectrode structure, honeycomb structure, an in-plane switchingstructure or other combined in-plane-switching and vertical switchingmay be used. Moreover, the invention may be implemented in passivematrix as well as active matrix electrophoretic displays. In fact, theinvention can be implemented in any bi-stable display that does notconsume power while the image substantially remains on the display afteran image update. Also, the invention is applicable to both single andmultiple window displays, where, for example, a typewriter mode exists.

While there has been shown and described what are considered to bepreferred embodiments of the invention, it will, of course, beunderstood that various modifications and changes in form or detailcould readily be made without departing from the spirit of theinvention. It is therefore intended that the invention not be limited tothe exact forms described and illustrated, but should be construed tocover all modifications that may fall within the scope of the appendedclaims.

1. A method for providing a set of voltage waveforms for updating atleast a portion of a bi-stable display in successive frame periods, themethod comprising: accessing data defining the set of voltage waveformsfor the successive frame periods; and generating the set of voltagewaveforms (600, 620, 640, 660; 700, 720, 740, 760; 800, 820, 840, 860)for driving the at least a portion of the bi-stable display (310) duringthe successive frame periods according to the accessed data; wherein:over a duration of the successive frame periods, each of the voltagewaveforms spans a first range of values; and at least one of thesuccessive frame periods is time-aligned with a data-dependent portionof each of the voltage waveforms that spans a second range of valuesthat is a subset of the first range of values.
 2. The method of claim 1,wherein: at least one other of the successive frame periods istime-aligned with a data-dependent portion of each of the voltagewaveforms that spans a third range of values that is a subset of thefirst range of values.
 3. The method of claim 2, wherein: the second andthird ranges of values are contiguous and span the first range ofvalues.
 4. The method of claim 1, wherein: a relatively shorter frameperiod (FT′) is used during the at least one of the successive frameperiods.
 5. The method of claim 1, wherein: the data-dependent portionof each of the voltage waveforms comprises a reset portion (R).
 6. Themethod of claim 1, wherein: the data-dependent portion of each of thevoltage waveforms comprises a drive portion (D, D1, D2).
 7. The methodof claim 1, wherein: the data-dependent portion of each of the voltagewaveforms comprises a first drive portion (D1), followed by a delay,followed by a second drive portion (D2).
 8. The method of claim 1,wherein: the bi-stable display comprises an electrophoretic display. 9.The method of claim 1, further comprising: lowering a supply voltage ofa voltage source used for the generating of the set of voltage waveformsduring the at least one of the successive frame periods, from a supplyvoltage associated with the first range of values to a supply voltageassociated with the second range of values.
 10. A program storage devicetangibly embodying a program of instructions executable by a machine toperform a method for providing a set of voltage waveforms for updatingat least a portion of a bi-stable display in successive frame periods,the method comprising: accessing data defining the set of voltagewaveforms for the successive frame periods; and generating the set ofvoltage waveforms (600, 620, 640, 660; 700, 720, 740, 760; 800, 820,840, 860) for driving the at least a portion of the bi-stable display(310) during the successive frame periods according to the accesseddata; wherein: over a duration of the successive frame periods, each ofthe voltage waveforms spans a first range of values; and at least one ofthe successive frame periods is time-aligned with a data-dependentportion of each of the voltage waveforms that spans a second range ofvalues that is a subset of the first range of values.
 11. The programstorage device of claim 10, wherein: at least one other of thesuccessive frame periods is time-aligned with a data-dependent portionof each of the voltage waveforms that spans a third range of values thatis a subset of the first range of values.
 12. The program storage deviceof claim 10, wherein: a relatively shorter frame period (FT′) is usedduring the at least one of the successive frame periods.
 13. The programstorage device of claim 10, wherein: the data-dependent portion of eachof the voltage waveforms comprises at least one of a reset portion (R)and a drive portion (D, D1, D2).
 14. The program storage device of claim10, wherein: the bi-stable display comprises an electrophoretic display.15. The program storage device of claim 10, wherein the method furthercomprises: lowering a supply voltage of a voltage source used for thegenerating of the set of voltage waveforms during the at least one ofthe successive frame periods, from a supply voltage associated with thefirst range of values to a supply voltage associated with the secondrange of values.
 16. An display device, comprising: a bi-stable display(310); and a control (100) for providing a set of voltage waveforms forupdating at least a portion of a bi-stable display (310) in successiveframe periods by: (a) accessing data defining the set of voltagewaveforms for the successive frame periods, and (b) generating the setof voltage waveforms (600, 620, 640, 660; 700, 720, 740, 760; 800, 820,840, 860) for driving the at least a portion of the bi-stable displayduring the successive frame periods according to the accessed data;wherein: over a duration of the successive frame periods, each of thevoltage waveforms spans a first range of values; and at least one of thesuccessive frame periods is time-aligned with a data-dependent portionof each of the voltage waveforms that spans a second range of valuesthat is a subset of the first range of values.
 17. The display device ofclaim 16, wherein: a relatively shorter frame period (FT′) is usedduring the at least one of the successive frame periods.
 18. The displaydevice of claim 16, wherein: the data-dependent portion of each of thevoltage waveforms comprises at least one of a reset portion (E) and adrive portion (D, D1, D2).
 19. The display device of claim 16, wherein:the bi-stable display comprises an electrophoretic display.
 20. Thedisplay device of claim 16, wherein: the control lowers a supply voltageof a voltage source used for the generating of the set of voltagewaveforms during the at least one of the successive frame periods, froma supply voltage associated with the first range of values to a supplyvoltage associated with the second range of values.
 21. A controller(100) comprising means for accessing data defining a set of voltagewaveforms (600, 620, 640, 660; 700, 720, 740, 760; 800, 820, 840, 860)for updating at least a portion of a bi-stable display (310) insuccessive frame periods and comprising an arithmetic logic circuitconfigured to generate the set of voltage waveforms (600, 620, 640, 660;700, 720, 740, 760; 800, 820, 840, 860) for driving the at least aportion of the bi-stable display during the successive frame periodsaccording to the accessed data; wherein: over a duration of thesuccessive frame periods, each of the voltage waveforms spans a firstrange of values; and at least one of the successive frame periods istime-aligned with a data-dependent portion of each of the voltagewaveforms that spans a second range of values that is a subset of thefirst range of values.